Analyzers have been provided for the detection of the concentration of liquid analytes using as analysis means, so-called dried test elements that contain within themselves the necessary reagents to permit such detection. Examples of such analyzers are described in U.S. Pat. Nos. 4,287,155 and 4,340,390. As is evident in the aforesaid U.S. Pat. No. 4,340,390, the preferred method of dispensing a small quantity of test fluid onto the test element is to transfer at least that quantity to be dispensed, such as by aspiration, from a first container into a temporary storage container called a disposable pipette or dispensing "tip". Examples are shown in U.S. Pat. No. 4,347,875. The fluid is transferred from the tip to the test element by pressurizing the tip by amounts effective to dispense a small quantity (typically 10 .mu.l) of liquid onto the test element.
Perfusion has been an occasional but persistent problem during the dispensing operation of the aforementioned analyzers. "Perfusion" refers to the movement of dispensed fluid up the exterior surface of the tip, rather than down onto the test element designed to receive the fluid, during the dispensing operation. Perfusion is a problem because fluid contacting the exterior surface of the tip alters the volume of fluid that is subsequently dispensed onto the test element. In some cases, perfusion can result in no fluid being dispensed on the test element. Studies have been conducted to locate the cause of the perfusion problem and provide a solution. Although it appears there may be a number of contributing factors which cause perfusion, it has been found that "flash" left at the end of the tip during the molding operation contributes significantly to the perfusion problem.
Flash is a small amount of plastic material that is left near the aperture of the tip due to wear of the molding tool or contaminates being present in the mold. Due to the small diameter of the tip aperture (e.g., about 0.5 mm), even a small amount of flash can divert the dispensed fluid and cause it to adhere to the side of the tip. Flash can also cause additional fluid to cling to the side of the tip during the withdrawal of the tip from the container holding the sample fluid, which causes additional fluid to be drawn to the side of the tip during the dispense operation.
Molding apparatus presently used to produce the tips are composed of an upper and lower mold plate. Only the upper mold plate has a cavity that defines the outer surface of the dispensing tip. A telescoping core pin fits into the cavity and extends from the upper plate into the lower mold plate when the mold is closed. The telescoping core pin defines the inner surface and aperture size of the dispensing tip. Surface contaminants on the upper or lower mold plate surface can prevent the surfaces from properly interfacing and cause a small amount of plastic material to extend radially from the tip aperture along the mold parting line, forming "radial" flash. "Axial" flash is formed due to the wear of the telescoping core pin which allows a small amount of plastic material to extend axially from the tip aperture into the lower mold plate.
The problem then, which is the basis for the present invention, is to eliminate flash from the dispensing tip in order to prevent perfusion.